The charge when one is in debt (5)
I believe the answer is:
debit
'the charge' is the definition.
(debit is a kind of charge)
'one is in debt' is the wordplay.
'one' becomes 'i' (Roman numeral).
'is in' means one lot of letters goes inside another.
'i' placed within 'debt' is 'DEBIT'.
'when' is the link.
(Other definitions for debit that I've seen before include "Take money away from" , "left-side entry" , "Record of a sum owed" , "Sum removed from a bank account" , "Opposite of credit" .)